Optical glass

ABSTRACT

An optical glass comprising, by mass %,
         12 to 40% of SiO 2 ,   15% or more but less than 42% of Nb 2 O 5 ,   2% or more but less than 18% of TiO 2 ,   (provided that Nb 2 O 5 /TiO 2  is over 0.6),   0.1 to 20% of Li 2 O,   0.1 to 15% of Na 2 O, and   0.1 to 25% of K 2 O,
 
and having an Abbe&#39;s number νd of 20 to 30, a ΔPg,F of 0.016 or less and a liquidus temperature of 1,200° C. or lower.

This application is a divisional of application Ser. No. 12/865,594filed Aug. 20, 2010 which in turn is the U.S. national phase ofInternational Application No. PCT/JP2009/051400 filed 22 Jan. 2009 whichdesignated the U.S. and claims priority to JP Patent Application No.2008-020724 filed 31 Jan. 2008, the entire contents of each of which arehereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an optical glass in a broad sense, and morespecifically, it relates to an optical glass having a high dispersionproperty and being suitable for correction of chromatic aberration, to apress-molding glass material and an optical element formed of the aboveglass each and processes for producing them, and to a process forproducing an optical element blank.

BACKGROUND ART

The correction of chromatic aberration requires a lens formed of ahigh-dispersion glass together with a lens formed of a low-dispersionglass. When aspherical lenses are employed as such lenses, ahigher-performance and compact optical system can be realized.

Mass-producing of such lenses requires a glass having a low glasstransition temperature, and a phosphate glass is available as a typicalexample thereof. In addition, a few silica-containing glasses have beenalso proposed as disclosed in JP 2004-161598A, Re-publishedWO2004/110942 and JP 2002-87841A.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

For high-order achromatism in an image-sensing optical system orprojection optical system, it is effective to use a lens formed of alow-dispersion glass and a lens formed of a high-dispersion glass incombination. Since, however, many glasses on the low-dispersion sidehave large partial dispersion ratios, it is more effective forhigh-order correction of chromatic aberration to combine a lens formedof such a glass with a lens formed of a glass that not only has a highdispersion property but also has a small partial dispersion ratio.High-dispersion phosphate glasses that are mainstream glasses forprecision press-molding have large partial dispersion ratios, and it isdifficult to make a phosphate glass suitable for the above object.

On the other hand, the silica-containing glasses disclosed in JP2004-161598A and Re-published WO2004/110942 have low glass stability,and they are unsuitable for mass-production since a crystal isprecipitated during stirring for obtaining a homogeneous molten glass orsince a crystal is precipitated during the casting of a molten glass toshape a glass.

Further, the silica-containing glass disclosed in JP 2002-87841A has alarge partial dispersion ratio, and it requires an improvement for useas an achromatic material of high order.

It is an object of this invention to provide an optical glass thatovercomes the above problems and also has a high dispersion property andthat is suitable for achromatism of high order and has excellent glassstability, a press-molding glass material formed of the above glass, anoptical element formed of the above glass, and process for producing anoptical element blank and an optical element.

Means to Solve the Problems

This invention provides, as means for overcoming the above problems,

(1) an optical glass comprising, by mass %,

12 to 40% of SiO₂,

15% or more but less than 42% of Nb₂O₅,

2% or more but less than 18% of TiO₂,

(provided that Nb₂O₅/TiO₂ is over 0.6),

0.1 to 20% of Li₂O,

0.1 to 15% of Na₂O, and

0.1 to 25% of K₂O,

the optical glass having an Abbe's number νd of 20 to 30, a ΔPg,F of0.016 or less and a liquidus temperature of 1,200° C. or lower,

(2) an optical glass as recited in the above (1), which contains, asoptional components,

0 to 10% of B₂O₃,

0 to 20% of ZrO₂,

0 to 22% of WO₃,

0 to 17% of CaO,

0 to 13% of SrO,

0 to 20% of BaO,

(provided that the total content of CaO, SrO and BaO is 0 to 25%),

0 to 13% of ZnO,

0 to 3% of La₂O₃,

0 to 3% of Gd₂O₃,

0 to 3% of Y₂O₃,

0 to 3% of Yb₂O₃,

0 to 10% of Ta₂O₅,

0 to 3% of GeO₂,

0 to 10% of Bi₂O₃, and

0 to 10% of Al₂O₃,

the total content of Nb₂O₅ and TiO₂ being 35 to 65%, the total contentof Li₂O, Na₂O and K₂O being 1 to 30%,

the optical glass having a refractive index nd of 1.82 or more but lessthan 1.87,

(3) an optical glass as recited in the above (1), which contains, asoptional components,

0 to 10% of B₂O₃,

0 to 20% of ZrO₂,

0 to 20% of WO₃,

0 to 13% of CaO,

0 to 13% of SrO,

0 to 20% of BaO,

(provided that the total content of CaO, SrO and BaO is 0 to 25%),

0 to 13% of ZnO,

0 to 3% of La₂O₃,

0 to 3% of Gd₂O₃,

0 to 3% of Y₂O₃,

0 to 3% of Yb₂O₃,

0 to 10% of Ta₂O₅,

0 to 3% of GeO₂,

0 to 10% of Bi₂O₃, and

0 to 10% of Al₂O₃,

the total content of Nb₂O₅ and TiO₂ being 35 to 60%, the total contentof K₂O being 0.1 to 15%, the total content of Li₂O, Na₂O and K₂O being 1to 25%, the optical glass having a refractive index nd of 1.87 to 1.90,

(4) an optical glass as recited in any one of the above (1) to (3),which contains Sb₂O₃ in an amount of 0 to 2% and SnO₂ in an amount of 0to 2%, these amounts being based on a glass composition excluding Sb₂O₃and SnO₂,

(5) an optical glass as recited in any one of the above (1) to (4),which has a partial dispersion ratio, Pg,F, of 0.580 to 0.620,

(6) an optical glass as recited in any one of the above (1) to (5),which has a glass transition temperature of less than 600° C.,

(7) a press-molding glass material formed of the optical glass recitedin any one of the above (1) to (6),

(8) an optical element formed of the optical glass recited in any one ofthe above (1) to (6),

(9) a process for producing an optical element blank, which comprisessoftening the glass material recited in the above (7) under heat andpress-molding the glass material,

(10) a process for producing an optical element blank, which comprisessupplying a molten glass to a press mold and press-molding the moltenglass,

wherein glass raw materials prepared so as to obtain the optical glassrecited in any one of the above (1) to (6) are melted under heat and theresultant molten glass is press-molded,

(11) a process for producing an optical element, which comprisesgrinding and polishing the optical element blank produced by the processrecited in the above (9) or (10),

(12) a process for producing an optical element, which comprises heatingthe press-molding glass material recited in the above (7) and precisionpress-molding it with a press mold,

(13) a process for producing an optical element as recited in the above(12), which comprises introducing the glass material into the press moldand heating said mold and the glass material together, and

(14) a process for producing an optical element as recited in the above(12), which comprises heating the glass material and introducing theglass material to a pre-heated press mold to carry out the precisionpress-molding.

Effect of the Invention

According to this invention, there can be provided an optical glass thathas a high dispersion property and is suitable for achromatism of highorder and that has excellent glass stability, a press-molding glassmaterial formed of the above glass and an optical element formed of theabove glass. Further, there can be provided a process for producing anoptical element blank formed of the above glass and a process forproducing an optical element.

PREFERRED EMBODIMENTS OF THE INVENTION Optical Glass

First, the optical glass of this invention will be explained.

In general, a high-dispersion glass exhibits positive anomalousdispersion. When the partial dispersion property of the high-dispersionglass can be brought close to a normal line in a partial dispersionratio Pg,f-Abbe's number νd diagram while maintaining the highdispersion property and keeping the partial dispersion ratio Pg,F small,there can be provided an optical glass material that is highly effectivefor high-order correction of chromatic aberration when combined with alens formed of a low-dispersion glass.

For making such a glass material real, the present inventors haveemployed, as a base, a silica-containing composition advantageous forbrining the partial dispersion property close to the normal line, andfor imparting high-refractivity high-dispersion properties, they haveintroduced Nb₂O₅ and TiO₂ as essential components.

On the basis of finding that the present inventors have acquired, Nb₂O₅works much better to inhibit the partial dispersion ratio than TiO₂. Itis hence decided that the partial dispersion ratio is to be controlledby adjusting the ratio of Nb₂O₅ and TiO₂.

When the moldability during the reheating and softening of a glass istaken into account, alkali metal components are introduced for impartingthe glass with the property of being softened at a low temperature,while it has been found that the glass stability is sharply decreasedwhen any one of Li₂O, Na₂O and K₂O weighs too much in the introductionof the alkali metal components. The present inventors have thereforesucceeded in remarkable improvement of the glass stability on the basisof a mixed alkali effect produced by making Li₂O, Na₂O and K₂Oco-present as glass components.

On the basis of the above findings, the components have been optimizedand this invention has been completed.

That is, the optical glass of this invention comprises, by mass %,

12 to 40% of SiO₂,

15% or more but less than 42% of Nb₂O₅,

2% or more but less than 18% of TiO₂,

(provided that Nb₂O₅/TiO₂ is over 0.6),

0.1 to 20% of Li₂O,

0.1 to 15% of Na₂O, and

0.1 to 25% of K₂O,

the optical glass having an Abbe's number νd of 20 to 30, a partialdispersion ratio of 0.580 to 0.620, a ΔPg,F of 0.016 or less and aliquidus temperature of 1,200° C. or lower.

The optical glass of this invention exhibits a high refractive index,for example, of 1.82 to 1.90, and an optical element formed of thisglass is effective for downsizing an optical system.

The above partial dispersion ratio Pg,F is expressed as (ng−nF)/(nF−nc)in which ng, nF and nc are refractive indices to g-ray, F-ray and c-ray.

When a partial dispersion ratio on a normal line that is the standard ofa normal partial dispersion glass in the partial dispersion ratioPg,f-Abbe's number νd diagram is expressed as Pg,F^((O)), Pg,F⁽⁰⁾ can beexpressed by the following equation using an Abbe's number νd.

Pg,F ⁽⁰⁾=0.6483−(0.0018×νd)

ΔPg,F is a deviation of a partial dispersion ratio from the above normalline and is expressed by the following equation.

$\begin{matrix}{{\Delta \; P_{g,F}} = {P_{g,F} - P_{g,F}^{(0)}}} \\{= {P_{g,F} + \left( {0.0018 \times {vd}} \right) - 0.6483}}\end{matrix}\quad$

In the present specification, a content by % and a total content by %stand for a content by mass % and a total content by mass % unlessotherwise specified, and a content ratio also means a mass ratio.

The optical glass of this invention is largely classified into thefollowing two embodiments.

The first embodiment is an optical glass containing, as optionalcomponents,

0 to 10% of B₂O₃,

0 to 20% of ZrO₂,

0 to 22% of WO₃,

0 to 17% of CaO,

0 to 13% of SrO,

0 to 20% of BaO,

(provided that the total content of CaO, SrO and BaO is 0 to 25%),

0 to 13% of ZnO,

0 to 3% of La₂O₃,

0 to 3% of Gd₂O₃,

0 to 3% of Y₂O₃,

0 to 3% of Yb₂O₃,

0 to 10% of Ta₂O₅,

0 to 3% of GeO₂,

0 to 10% of Bi₂O₃, and

0 to 10% of Al₂O₃,

the total content of Nb₂O₅ and TiO₂ being 35 to 65%, the total contentof Li₂O, Na₂O and K₂O being 1 to 30%,

the optical glass having a refractive index nd of 1.82 or more but lessthan 1.87.

The second embodiment is an optical glass having a higher refractiveindex than the first embodiment and contains, as optional components,

0 to 10% of B₂O₃,

0 to 20% of ZrO₂,

0 to 20% of WO₃,

0 to 13% of CaO,

0 to 13% of SrO,

0 to 20% of BaO,

(provided that the total content of CaO, SrO and BaO is 0 to 25%),

0 to 13% of ZnO,

0 to 3% of La₂O₃,

0 to 3% of Gd₂O₃,

0 to 3% of Y₂O₃,

0 to 3% of Yb₂O₃,

0 to 10% of Ta₂O₅,

0 to 3% of GeO₂,

0 to 10% of Bi₂O₃,

0 to 10% of Al₂O₃,

the total content of Nb₂O₅ and TiO₂ being 35 to 60%, the total contentof K₂O being 0.1 to 15%, the total content of Li₂O, Na₂O and K₂O being 1to 25%, the optical glass having a refractive index nd of 1.87 to 1.90.

In the optical glasses according to the first and second embodiments,Sb₂O₃ and SnO₂ can be added in an amount of 0 to 2% each based on aglass composition excluding Sb₂O₃ and SnO₂.

Functions of the above components and reasons for limitations ofcompositional ranges thereof will be explained.

SiO₂ is a glass network-forming oxide and is an essential component formaintaining the stability of a glass and the shapeability of a moltenglass. When the content thereof is less than 12%, the glass stability isdecreased and the chemical durability is degraded. Further, theviscosity of the glass during the formation of a molten glass becomestoo low, and the shapeability is decreased. When the content thereofexceeds 40%, the liquidus temperature and the glass transitiontemperature increase, and the devitrification resistance and themeltability are also degraded. Further, it is difficult to realize adesired Abbe's number νd. The content of SiO₂ is therefore adjusted to12 to 40%. The content of SiO₂ is preferably in the range of 15 to 35%,more preferably in the range of 18 to 33%, still more preferably in therange of 20 to 30%, yet more preferably in the range of 22 to 28%.

Nb₂O₅ is an essential component that works to increase the refractiveindex and improve the devitrification resistance by decreasing theliquidus temperature. It is also a component that works to bring thepartial dispersion property close to the normal line amonghigh-refractivity-imparting components, that is, a component that worksto bring ΔPg,F close to zero. When the content thereof is less than 15%,there is caused a problem that it is difficult to maintain the desiredrefractive index, etc., and it is difficult to bring the partialdispersion property close to the normal line. When the content thereofis 42% or more, the liquidus temperature increases, and thedevitrification resistance is degraded. The content of Nb₂O₅ istherefore adjusted to 15% or more but less than 42%. The lower limit ofcontent of Nb₂O₅ is preferably 18%, more preferably 20%, still morepreferably 22%, yet more preferably 25%, and the upper limit thereof ispreferably 41.5%, more preferably 41%.

TiO₂ is an essential component effective for increasing the refractiveindex and improving the devitrification resistance and chemicaldurability. When the content thereof is less than 2%, the above effectsare not produced. When it is 18% or more, it is difficult to realize thedesired Abbe's number νd. The content of TiO₂ is therefore adjusted to2% or more but less than 18%. Preferably, the content of TiO₂ is in therange of 3% or more but less than 16% for the optical glass according tothe first embodiment or in the range of 2% or more but less than 12% forthe optical glass according to the second embodiment. More preferably,it is in the range of 4% or more but less than 14% for the optical glassaccording to the first embodiment or in the range of 3% or more but lessthan 12% for the optical glass according to the second embodiment. Stillmore preferably, it is in the range of 5% or more but less than 12% forthe optical glass according to the first embodiment or in the range of4% or more but less than 12% for the optical glass according to thesecond embodiment. Yet more preferably, it is it is in the range of 6%or more but less than 12% for the optical glass according to the firstembodiment or in the range of 5% or more but less than 12% for theoptical glass according to the second embodiment. In the optical glassaccording to the second embodiment, further more preferably, it is inthe range of 6% or more but less than 12%.

For controlling ΔPg,F, the ratio of the content of Nb₂O₅ to the contentof TiO₂, Nb₂O₅/TiO₂, plays an important role. When the above ratio is0.6 or less, the partial dispersion ratio and ΔPg,F increase, and thechromatic correction effect of high order decreases. Therefore, theratio of the content of Nb₂O₅ to the content of TiO₂, Nb₂O₅/TiO₂ isadjusted to over 0.6. The above ratio Nb₂O₅/TiO₂ is preferably in therange of 0.70 or more, more preferably in the range of 0.80 or more,still more preferably in the range of 0.90 or more, yet more preferablyin the range of 1.0 or more.

Li₂O works to improve the meltability and decrease the glass transitiontemperature. Of the alkali metal components, Li₂O works much better todecrease the glass transition temperature in particular, and it is alsoa component that can relatively maintain a high refractive index.Further, thanks to the mixed alkali effect produced by the co-presencethereof with Na₂O and K₂O, the glass stability is improved. When thecontent of Li₂O is less than 0.1%, the above effects cannot be obtained.When it exceeds 20%, the liquidus temperature increases and thedevitrification resistance decreases. The content of Li₂O is thereforeadjusted to 0.1 to 20%. The content of Li₂O is preferably in the rangeof 0.1 to 17, more preferably in the range of 0.1 to 15%. In the opticalglass according to the first embodiment, the content of Li₂O is stillmore preferably in the range of 1 to 10%, yet more preferably in therange of 1 to 5%. In the second embodiment, it is still more preferablyin the range of 1 to 12%, yet more preferably in the range of 1 to 10%.

Na₂O works to improve the meltability and decrease the glass transitiontemperature. Further, it works to remarkably improve the glass stabilitytogether with Li₂O thanks to the mixed alkali effect. When the contentof Na₂O is less than 0.1%, the above effects cannot be produced. When itexceeds 15%, the liquidus temperature increases, and the devitrificationresistance decreases. The content of Na₂O is therefore adjusted to 0.1to 15%. The content of Na₂O is preferably in the range of 0.1 to 12%,more preferably in the range of 0.5 to 10%.

In the optical glass according to the first embodiment, the content ofNa₂O is still more preferably in the range of 0.5 to 9%, yet morepreferably in the range of 0.5 to 8%. In the optical glass according tothe second embodiment, the content of Na₂O is still more preferably inthe range of 0.5 to 7%, yet more preferably in the range of 0.5 to 5%.

K₂O also works to improve the meltability and decrease the glasstransition temperature. It also works to remarkably improve the glassstability together with Li₂O and Na₂O thanks to the mixed alkali effect.When the content of K₂O is less than 0.1%, the above effects cannot beproduced. When the content of K₂O exceeds 25%, the liquidus temperatureincreases, and the devitrification resistance decreases. The content ofK₂O is therefore adjusted to 0.1 to 25%.

In the optical glass according to the first embodiment, the content ofK₂O is preferably in the range of 0.1 to 22%, more preferably in therange of 0.5 to 20%, still more preferably in the range of 0.5 to 17%,yet more preferably in the range of 0.5 to 15%.

In the optical glass according to the second embodiment, the content ofK₂O is preferably in the range of 0.1 to 15%, more preferably in therange of 0.1 to 12%, still more preferably in the range of 0.5 to 10%,yet more preferably in the range of 0.5 to 7%, further more preferablyin the range of 0.5 to 5%.

The Abbe's number νd of the optical glass of this invention (includingthe optical glasses according to the first and second embodiments) is 20to 30. For further improving the devitrification resistance andrealizing the desired partial dispersion property, the Abbe's number νdis preferably in the range of 21 to 29, more preferably in the range of22 to 29.

The optical glass of this invention has ΔPg,F of 0.016 or less. Formaking it easier to improve the various properties above, ΔPg,F ispreferably 0.015 or less, more preferably 0.014 or less, still morepreferably 0.013 or less, yet more preferably 0.012 or less. The lowerlimit of ΔPg,F is not specially limited, while it is generally 0 ormore, and for making it easier to improve the various properties above,it is preferably 0.001 or more, more preferably 0.002 or more, stillmore preferably 0.005 or more, yet more preferably 0.007 or more.

For making it easier to improve the various properties above,preferably, the partial dispersion ratio Pg,F is adjusted to 0.580 to0.620. Pg,F is more preferably in the range of 0.585 to 0.620, stillmore preferably in the range of 0.590 to 0.619, yet more preferably inthe range of 0.595 to 0.618, further more preferably in the range of0.600 to 0.618.

The optical glass of this invention has a liquidus temperature of 1,200°C. or lower and is excellent in stability. The liquidus temperature ispreferably in the range of 1,180° C. or lower, more preferably in therange of 1,160° C. or lower.

Optional components will be explained below.

B₂O₃ is an oxide for forming a glass network and works to improve themeltability and decrease the liquidus temperature, and moreover, it is acomponent effective for realizing a low dispersion property. However,when it is introduced in an amount exceeding 10%, the refractive indexis decreased, and the chemical durability is degraded. The content ofB₂O₃ is therefore adjusted to 0 to 10%. The content of B₂O₃ ispreferably in the range of 0 to 8%, more preferably in the range of 0 to7%, still more preferably in the range of 0 to 6%, yet more preferablyin the range of 0 to 5%.

ZrO₂ works to increase the refractive index and improve the chemicaldurability. However, when the content thereof exceeds 20%, thedevitrification resistance is decreased, and the glass transitiontemperature is increased. The content of ZrO₂ is therefore adjusted to 0to 20%. The content of ZrO₂ is preferably in the range of 0 to 16%, morepreferably in the range of 0 to 14%, still more preferably in the rangeof 0 to 12%, yet more preferably in the range of 0 to 10%.

WO₃ works to increase the refractive index nd, decrease the liquidustemperature and improve the devitrification resistance. However, whenthe content thereof in the optical glass according to the firstembodiment exceeds 22%, or when the content thereof in the optical glassaccording to the second embodiment exceeds 20%, the liquidus temperatureis increased and the devitrification resistance is decreased. Further,the coloring the glass is intensified. The content of WO₃ in the opticalglass according to the first embodiment is adjusted to 0 to 22%, and thecontent of WO₃ in the optical glass according to the second embodimentis adjusted to 0 to 20%. The content of WO₃ in the optical glassaccording to the first embodiment is preferably in the range of 0 to20%, more preferably in the range of 0 to 17%, still more preferably inthe range of 1 to 15%, yet more preferably in the range of 1 to 12%. Thecontent of WO₃ in the optical glass according to the second embodimentis preferably in the range of 0 to 17%, more preferably in the range of0 to 15%, still more preferably in the range of 1 to 12%, yet morepreferably in the range of 1 to 10%.

CaO works to improve the meltability and increase the lighttransmittance. Further, when it is introduced into the glass in the formof a carbonate material or nitrate material, a defoaming effect can bealso produced. However, when the content thereof in the optical glassaccording to the first embodiment exceeds 17%, or when the contentthereof in the optical glass according to the second embodiment exceeds13%, the liquidus temperature is increased, and the devitrificationresistance is increased. Further, the refractive index is alsodecreased. Therefore, the content of CaO in the optical glass accordingto the first embodiment is adjusted to 0 to 17% and the content of CaOin the optical glass according to the second embodiment is adjusted to 0to 13%. The content of CaO in the first embodiment is preferably in therange of 0 to 15%, more preferably in the range of 0 to 12%, still morepreferably in the range of 0 to 10%, yet more preferably in the range of0 to 8%. The content of CaO in the second embodiment is preferably inthe range of 0 to 12%, more preferably in the range of 0 to 10%, stillmore preferably in the range of 0 to 7%, yet more preferably in therange of 0 to 5%.

SrO also works to improve the meltability and increase the lighttransmittance. Further, when it is introduced into the glass in the formof a carbonate material or a nitrate material, a defoaming effect can bealso produced. However, when the content thereof exceeds 13%, theliquidus temperature is increased, and the devitrification resistance isdecreased. Further, the refractive index is decreased. The content ofSrO is therefore adjusted to 0 to 13%. The content of SrO is preferablyin the range of 0 to 12%, more preferably in the range of 0 to 10%,still more preferably in the range of 0 to 7%, yet more preferably inthe range of 0 to 5%.

BaO also works to improve the meltability and increase the lighttransmittance. Further, when it is introduced into the glass in the formof a carbonate material or a nitrate material, a defoaming effect can bealso produced. However, when the content thereof exceeds 20%, theliquidus temperature is increased, and the devitrification resistance isdecreased. Further, the refractive index is decreased. The content ofBaO is therefore adjusted to 0 to 20%. The content of BaO is preferablyin the range of 0 to 17%, and in the optical glass according to thefirst embodiment, it is more preferably in the range of 0 to 15%, stillmore preferably in the range of 0 to 12%, yet more preferably in therange of 0 to 10%. In the optical glass according to the secondembodiment, the content of BaO is more preferably in the range of 1 to15%, still more preferably in the range of 2 to 12%, yet more preferablyin the range of 3 to 10%.

For preventing the liquidus temperature from increasing and forimproving the devitrification resistance, desirably, the total contentof CaO, SrO and BaO is adjusted to 0 to 25%. The total content of CaO,SrO and BaO is more preferably in the range of 1 to 22%, still morepreferably in the range of 2 to 20%, yet more preferably in the range of3 to 17%, further more preferably in the range of 5 to 15%.

ZnO also works to improve the meltability and increase the lighttransmittance. Further, when it is introduced into the glass in the formof a carbonate material or a nitrate material, a defoaming effect can bealso produced. However, when the content thereof exceeds 13%, theliquidus temperature is increased, and the devitrification resistance isdecreased. Further, the refractive index is decreased. The content ofZnO is therefore adjusted to 0 to 13%. The content of ZnO is preferablyin the range of 0 to 12%, more preferably in the range of 0 to 10%,still more preferably in the range of 0 to 7%, yet more preferably inthe range of 0 to 5%.

La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ work to increase the refractive index andchemical durability. However, when any one of these is introduced in anamount exceeding 3%, the liquidus temperature is increased, and thedevitrification resistance is decreased. The content of each of La₂O₃,Gd₂O₃, Y₂O₃ and Yb₂O₃ is therefore adjusted to 0 to 3%. The content ofeach of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is preferably in the range of 0 to2%, more preferably in the range of 0 to 1%. Still more preferably, noneof La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is introduced.

Ta₂O₅ also works to increase the refractive index and improve thechemical durability. However, when it is introduced in an amountexceeding 10%, the liquidus temperature is increased, and thedevitrification resistance is decreased. The content of Ta₂O₅ istherefore adjusted to 0 to 10%. The content of Ta₂O₅ is preferably inthe range of 0 to 7%, more preferably in the range of 0 to 5%.

GeO₂ is a network-forming oxide, and also works to increase therefractive index. Since, however, it is an expensive component, thecontent of GeO₂ is adjusted to 0 to 3%, more preferably, to 0 to 2%.Still more preferably, no GeO₂ is introduced.

Bi₂O₃ works not only to increase the refractive index but also toimprove the glass stability. However, when it is introduced in an amountexceeding 10%, the coloring of the glass is intensified. The content ofBi₂O₃ is therefore adjusted to 0 to 10%, more preferably, to 0 to 5%.The content of Bi₂O₃ in the optical glass according to the firstembodiment is still more preferably in the range of 0 to 4%, and thecontent of Bi₂O₃ in the optical glass according to the second embodimentis still more preferably in the range of 0 to 3%.

Al₂O₃ works to improve the glass stability and chemical durability whenintroduced in a small amount. However, when it is introduced in anamount exceeding 10%, the liquidus temperature is increased, and thedevitrification resistance is decreased. The content of Al₂O₃ istherefore adjusted to 0 to 10%, more preferably, to 0 to 5%, still morepreferably, to 0 to 3%.

In the optical glass according to the first embodiment, the totalcontent of Nb₂O₅ and TiO₂ is adjusted to 35 to 65%, preferably, to 38 to62%, more preferably, to 40 to 62%, still more preferably, to 43 to 60%,yet more preferably, to 45 to 58%.

In the optical glass according to the second embodiment, the totalcontent of Nb₂O₅ and TiO₂ is adjusted to 30 to 60%, preferably, to 33 to59%, more preferably, to 35 to 58%, still more preferably, to 38 to 57%,yet more preferably, to 40 to 55%.

In any one of the optical glasses according to the first and secondembodiments, when the total content of Nb₂O₅ and TiO₂ is too small, itis difficult to realize the predetermined optical properties. When theabove total content is too large, the liquidus temperature increases,and the devitrification resistance decreases.

In the optical glass according to the first embodiment, the totalcontent of Li₂O, Na₂O and K₂O is adjusted to 1 to 30%, preferably, to 2to 27%, more preferably, to 3 to 25%, still more preferably, to 4 to22%, yet more preferably, to 5 to 20%.

In the optical glass according to the second embodiment, the totalcontent of Li₂O, Na₂O and K₂O is adjusted to 1 to 25%, preferably, to 2to 22%, more preferably, to 3 to 20%, still more preferably, to 4 to18%, yet more preferably, to 5 to 15%.

In any one of the optical glasses according to the first and secondembodiments, when the total content of Li₂O, Na₂O and K₂O is too small,the glass transition temperature increases, and the meltabilitydecreases. When the above total content is too large, the liquidustemperature increases, and the devitrification resistance decreases.

The optical glass according to the first embodiment has a highrefractive index nd of 1.82 or more but less than 1.87, preferably 1.82to 1.865, more preferably 1.82 to 1.860, while this refractive index isrelatively low for the optical glass of this invention.

On the other hand, the optical glass according to the second embodimenthas a refractive index nd of 1.87 to 1.90, preferably 1.87 to 1.895,more preferably 1.87 to 1.89, or it corresponds to a glass having arelatively high refractive index for the optical glass of thisinvention.

The glass of this invention is not required to contain components suchas Lu and Hf. Since both Lu and Hf are also expensive components, it ispreferred to limit the content of each of Lu₂O₃ and HfO₂ to 0 to 1%, andit is more preferred to limit the content of each to 0 to 0.5%. It isparticularly preferred to introduce none of Lu₂O₃ and HfO₂.

In view of detrimental effects on the environment, it is also preferredto introduce none of As, Pb, U, Th, Te and Cd.

Further, for taking advantage of the excellent optical transmittance ofthe glass, it is preferred to introduce none of substances that causecoloring, such as Cu, Cr, V, Fe, Ni, Co, etc.

In the optical glass of this invention, Sb₂O₃ and SnO₂ can be added inan amount of 0 to 2% each based on a glass composition excluding Sb₂O₃and SnO₂. These additives work as a refiner, and besides this, Sb₂O₃ caninhibit the coloring of the glass caused by inclusion of impurities suchas Fe, etc. The amount of each of Sb₂O₃ and SnO₂ based on a glasscomposition excluding these is preferably 0 to 1%, more preferably 0 to0.5%.

The glass transition temperature of the glass of this invention ispreferably less than 600° C., more preferably 590° C. or lower, stillmore preferably 580° C. or lower. Having the above low glass transitiontemperature, the optical glass of this invention is not only suitablefor precision press-molding, but also excellent in moldability duringits molding by re-heating and softening of the glass. Since the glasstransition temperature is low as described above, the heatingtemperature during molding can be as well kept relatively low.Therefore, a chemical reaction between the glass and a mold such as apress mold, or the like does not easily take place, so that glass shapedmaterials having clean and smooth surfaces can be shaped. Further, thedeterioration of the mold can be suppressed.

The above optical glass can be obtained by weighing and formulatingoxides, carbonates, sulfate, nitrates, hydroxides, etc., as rawmaterials so as to obtain an intended glass composition, fully mixingthe formulated raw materials to prepare a batch mixture, melting thebatch mixture under heat in a melting vessel, carrying out defoaming andstirring to prepare a homogeneous and bubbles-free molten glass, andshaping the molten glass. Specifically, it can be produced by employinga known melting method.

[Press-Molding Glass Material]

The press-molding glass material of this invention will be explainedbelow.

The press-molding glass material of this invention is characteristicallyformed of the above optical glass of this invention.

The above glass material means a glass lump to be press-molded. Examplesof the glass material include glass lumps having masses equivalent tothe masses of press-molded products, such as a precision press-moldingpreform, a press-molding glass gob of an optical element blank.

The above examples of the glass material will be explained below.

The precision press-molding preform (to be sometimes simply referred toas “preform” hereinafter) means a pre-shaped glass material that is tobe precision press-molded by heating. The above precision press-moldingis also called optics molding as is well known, and is a method in whichthe optical-function surface of an optical element is formed bytransferring the form of molding surface of a press mold. Theoptical-function surface refers to a surface that refracts, reflects ordiffracts light to be controlled or allows light to be controlled toenter or go out in an optical element, and a lens surface of a lenscorrespond to the optical-function surface.

For preventing a reaction and fusing between a glass and the moldingsurface of a press mold and at the same time ensuring that the glassexcellently extends along the molding surface, preferably, the preformis surface-coated with a release film. The type of the release filmincludes,

-   -   noble metal (platinum, platinum alloy),    -   oxides (oxides of Si, Al, Zr, La, Y),    -   nitrides (nitrides of B, Si, Al), and    -   carbon-containing film.

The carbon-containing film is preferably a film containing carbon as amain component (a film in which the carbon content is larger than thecontent of any other element when element contents in the film areexpressed by atomic %). Specifically, a carbon film, a hydrocarbon film,etc., can be given as examples. The method for forming acarbon-containing film can be selected from known methods using a carbonmaterial, such as a vacuum vapor deposition method, a sputtering method,an ion plating method, etc., and known methods of thermal decomposition,etc., using a material gas such as hydrocarbon. The other films can beformed by employing a vapor deposition method, a sputtering method, anion plating method, a sol-gel method, etc.

The production of the preform is carried out as follows.

The first production example refers to a method in which a molten glasslump having a predetermined weight is separated from a molten glass andcooled to shape a preform having a mass equivalent to that of the moltenglass lump. For example, glass raw materials are melted, refined andhomogenized to prepare a homogeneous molten glass, and the molten glassis caused to flow out of a temperature-adjusted flow nozzle or flow pipemade of platinum or platinum alloy. When a preform of a small size or aspherical preform is shaped, molten glass is caused to drop as a moltenglass drop having a desired mass from the flow nozzle, and it isreceived with a preform shaping mold and shaped into a preform.Alternatively, a molten glass drop having a desired mass is caused todrop from the flow nozzle into liquid nitrogen, etc., to shape apreform. When a preform of an intermediate or large size is produced,molten glass flow is caused to flow downward from the flow pipe, theforward end of the molten glass flow is received with a preform shapingmold, a narrow portion is formed in the molten glass flow between thenozzle and the preform shaping mold, then, the preform shaping mold issharply moved vertically downward to separate the molten glass flow inthe narrow portion, and a separated molten glass lump having a desiredmass is received with a receiving member and shaped into a preform.

For producing a preform having a smooth surface free of scratches,soiling, creases, an altered surface, etc., for example, a preformhaving a free surface, there is employed a method in which a moltenglass lump is shaped into a preform while it is caused to float above apreform shaping mold by applying a gas pressure to it, or a method inwhich molten glass drops are introduced into a medium that is a liquidprepared by cooling a substance that is a gas at room temperature underatmospheric pressure, such as liquid nitrogen.

When a molten glass lump is shaped into a preform while it is caused tofloat, a gas (to be referred to as “floating gas”) is blown to themolten glass lump to exert a gas pressure thereon upwardly. In thiscase, when the viscosity of the molten glass lump is too low, floatinggas enters the glass and remains in the glass in the form of bubbles.However, when the viscosity of the molten glass lump is adjusted to 3 to60 dPa·s, the glass lump can be caused to float without the entering ofany floating gas into the glass.

The gas used for applying the floating gas to a preform includes air, N₂gas, O₂ gas, Ar gas, He gas, water vapor, etc. The gas pressure is notspecially limited so long as the preform can be caused to float withoutcontacting the shaping mold surface, etc.

Since many precision press-molded products (e.g., an optical element)produced from preforms have axes of rotational symmetry, preformsdesirably have forms having axes of rotational symmetry. Specificexamples of the forms thereof include the form of a sphere and a formhaving one axis of rotation symmetry. The form having one axis ofrotational symmetry includes forms each having a smooth outline free ofa corner or a dent in a cross section including the above axis ofrotational symmetry, such as a form having an outline that is an ellipsewhose minor axis corresponds to the axis or rotational symmetry in theabove cross section, and a form obtained by flattening a sphere (a formshaped by determining one axis passing the center of a sphere andshrinking the sphere in the direction of the above axis).

The second production example of the preform refers to a method in whicha homogeneous molten glass is cast into a mold, then, a shaped materialis annealed to remove a strain and cut or split to predetermineddimensions or form, a plurality of glass pieces are thereby produced,and the glass pieces are polished to smoothen their surfaces and formpreforms having a predetermined mass each. The thus-produced preformsare preferably used after a carbon-containing film is coated on thesurface of each.

A press-molding glass gob of an optical element blank as a glassmaterial refers to a glass lump that is used when an optical elementblank to be completed into an optical element by grinding and polishingis press-molded. The optical element blank has a form obtained by addinga processing margin to be removed by grinding and polishing to the formof an optical element.

The production example of the above glass gob refers to a method inwhich a homogeneous molten glass is cast into a mold, then, a shapedmaterial is annealed to remove a strain and cut or split topredetermined dimensions and form to prepare a plurality of glasspieces, and each glass piece is barrel-polished to round edges of eachglass piece so that the mass of the glass gob is adjusted so as to beequivalent to the mass of an optical element blank. The gob after thebarrel polishing has a roughened surface, which is a surface to which arelease agent in the form of a powder for press-molding can be easilyapplied.

In the second glass gob production example, the forward end of a moltenglass that is flowing out is received with a gob-shaping mold, a narrowportion is formed in the middle of the molten glass flow, and thegob-shaping mold is rapidly moved vertically downward to separate amolten glass in the narrow portion by means of a surface tension. Inthis manner, a molten glass lump having a desired mass is obtained, andthe glass lump is shaped while a gas pressure is applied to the glassupwardly by ejecting a gas to the glass. The thus-obtained glass lump isannealed and then barrel-polished to obtain a glass gob having a desiredmass.

[Optical Element]

The optical element of this invention will be explained below. Theoptical element of this invention is characteristically formed of theabove optical glass of this invention. Specifically, examples thereofinclude an aspherical lens, a spherical lens, or lenses such as aplano-concave lens, a plano-convex lens, a biconcave lens, a biconvexlens, a convex meniscus lens, a concave meniscus lens, etc., amicrolens, a lens array, a lens with a diffraction grating, a prism, aprism with a lens function, etc. The surface of the optical element maybe provided with an anti-reflection film, a partial reflecting filmhaving selectivity to a wavelength, etc.

Since the optical element of this invention is formed of the glasshaving a high-dispersion property but having a small partial dispersionratio, it can perform chromatic correction of high order when combinedwith an optical element formed of other glass. Further, since theoptical element of this invention is formed of the glass having a highrefractive index, an image-sensing optical system and a projectoroptical system can be downsized when it is used in such optical systems.

[Process for Producing Optical Element Blank]

The process for producing an optical element blank, provided by thisinvention, will be explained below.

The first process for the production of an optical element blank refersto a process of softening the above press-molding glass material of thisinvention under heat and press-molding it. As described above, a moldrelease agent in the form of a powder such as boron nitride is uniformlyapplied to the surface of the glass material, the glass material isplaced on a refractory vessel, introduced into a heating furnace andheated until the glass is softened, and then it is introduced into apress mold and press-molded. Then, a press-molded product is taken outof the mold, and annealed to remove a strain and adjust opticalproperties such as a refractive index, etc., such that they have desiredvalues.

The second process for the production of an optical element blank refersto a method of supplying a molten glass to a press mold to shape it intoan optical element blank, in which glass raw materials prepared forobtaining the above optical glass of this invention are melted underheat and a molten glass by the melting is press-molded. First, ahomogenized molten glass is caused to flow onto a lower mold membershaping surface having a powder mold release agent such as boronnitride, etc., uniformly applied thereon, and the molten glass flowwhose lower end portion is supported with the lower mold member is cutwith cutting blades called shears. In this manner, a molten glass lumphaving a desired mass is obtained on the molding surface of the lowermold member. Then, the lower mold member with the molten glass lump onit is carried to a place vertically right below an upper mold memberstanding by in other position, and the molten glass lump is pressed withthe upper mold member and the lower mold member to shape it in the formof an optical element blank. Then, press-shaped product is taken out ofthe mold and annealed to remove a strain and adjust optical propertiessuch as a refractive index, etc., such that they have desired values.

The above two production processes can be both carried out inatmosphere. Further, with regard to shaping conditions, a materialquality of the press mold, a furnace for softening under heat and avessel on which to place a glass gob for softening under heat, knownconditions and known tools, etc., can be employed.

[Process for Producing Optical Element]

The process for the production of an optical element, provided by thisinvention, will be explained below.

The first process for the production of an optical element, provided bythis invention, comprises grinding and polishing an optical elementblank produced by the above process of this invention. For grinding andpolishing, known methods can be employed.

The second process for the production of an optical element, provided bythis invention, comprises heating the above press-molding glass materialof this invention and precision press-molding it with a press mold. Theabove glass material refers to a preform.

The step of heating the press mold and the preform and pressing ispreferably carried out in a non-oxidizing gas atmosphere such as anatmosphere containing nitrogen gas or a gas mixture of nitrogen gas withhydrogen gas, for preventing the oxidation of molding surface of thepress mold or a release film provided on the above molding surface. Inthe non-oxidizing gas atmosphere, a carbon-containing film coating thepreform surface is not oxidized and remains on the surface of a moldedproduct obtained by the precision press-molding. This film is to befinally removed, and for relatively easily and completely removing thecarbon-containing film, the precision press-molded product can be heatedin an oxidizing atmosphere, e.g., in the atmosphere. The removal of thecarbon-containing film by oxidation is required to be carried out at atemperature at which the precision press-molded product is not deformedunder heat. Specifically, it is preferably carried out in a temperaturerange below the glass transition temperature.

The precision press-molding uses a press mold having molding surfacesthat are beforehand highly accurately processed to have a desired formeach, and a release film may be formed on the molding surface forpreventing the fusion of a glass during pressing. The release filmincludes a carbon-containing film, a nitride film and a precious metalfilm, and as a carbon-containing film, a hydrogenated carbon film, acarbon film, etc., are preferred. In the precision press-molding, apreform is fed between a pair of an upper mold member and a lower moldmember whose molding surfaces are accurately processed in form and whichface each other, then, both the mold and the preform are heated to atemperature corresponding to a glass viscosity of 10⁵ to 10⁹ dPa·s tosoften the preform, and the preform is press-molded, whereby the form ofmolding surfaces of the mold can be precisely transferred to the glass.

Alternatively, a preform that is temperature-increased beforehand to atemperature corresponding to a glass viscosity of 10⁴ to 10⁸ dPa·s isfed between a pair of an upper mold member and a lower mold memberhaving molding surfaces accurately processed in form and facing eachother, and the preform is press-molded, whereby the form of moldingsurfaces of the mold can be precisely transferred to the glass.

The pressure and time period for the pressing can be determined asrequired by taking account of the viscosity of a glass, etc., and forexample, the pressure can be set at approximately 5 to 15 MPa and thepressing time period can be set for 10 to 30 seconds. The pressconditions such as a time period for the pressing and the pressure canbe determined as required in well-known ranges depending upon the formand dimensions of a molded product.

Thereafter, the mold and the precision press-molded product are cooled,and preferably, when the temperature thereof reaches a strain point orlower, the precision press-molded product is separated from the mold andtaken out. In addition, for precisely adjusting optical properties todesired values, conditions for annealing the molded product, such as anannealing speed, etc., during cooling can be adjusted as required.

The above second process for the production of an optical element can belargely classified into the following two methods. The first processrefers to an optical element production process in which the glassmaterial is introduced into a press mold and the mold and the glassmaterial are heated together, and this process is recommendable whenimportance is attached to improvements in molding accuracy such assurface accuracy, eccentricity accuracy, etc. The second process refersto an optical element production process in which the glass material isheated, introduced into a pre-heated press mold and press-molded, andthis process is recommendable when importance is attached toimprovements in productivity.

In addition, the optical element of this invention can be also producedwithout carrying out a press-molding step. For example, the opticalelement can be obtained by casting a homogeneous molten glass into amold to form a glass block, annealing it to remove a strain and adjustoptical properties by adjusting annealing conditions such that therefractive index of the glass becomes a desired value, then, cutting orsplitting the glass block to prepare a glass piece and further grindingand polishing it to complete the optical element.

EXAMPLES

This invention will be more specifically explained with reference toExamples hereinafter, while this invention shall not be limited by theseExamples.

Example 1

Oxides, carbonates, sulfates, nitrates, hydroxides, etc., correspondingto raw materials for introducing components so as to obtain a glasscomposition shown in Table 1 or 2 were employed, the raw materials wereweighed and fully mixed to prepare a formulated raw material, and it wasplaced in a platinum crucible and melted under heat. After the melting,a molten glass was cast into a mold, and gradually cooled to atemperature around its glass transition temperature. Then, the glass wasplaced in an annealing furnace, annealed in a glass transitiontemperature range for approximately 1 hour and then allowed to cool inthe furnace to room temperature. In this manner, optical glasses Nos. 1to 17 were obtained. The glass Nos. 1 to 11 in Table 1 are glassesaccording to the first embodiment, and the glass Nos. 12 to 17 areglasses according to the second embodiment.

In the thus-obtained optical glasses, no crystal observable through amicroscope precipitated.

Table 3 shows various properties of the thus-obtained optical glasses.

The optical glasses were measured for properties by the followingmethods.

(1) Refractive Indices nd, ng, nF and nc and Abbe's Number νd

A glass obtained by decreasing temperature at a temperature decreaserate of −30° C./hour was measured for refractive indices nd, ng, nF andnc and an Abbe's number νd according to the refractive index measurementmethod of Japan Optical Glass Industrial Society Standard.

(2) Liquidus Temperature LT

A glass was placed in a furnace that was set under heat at apredetermined temperature for 2 hours, and cooled, and then, the glasswas internally observed through an optical microscope of 100magnifications. A liquidus temperature was determined on the basis ofwhether or not a crystal existed.

(3) Glass Transition Temperature Tg

Measured with a differential scanning calorimeter (DSC) at a temperatureelevation rate of 10° C./minute.

(4) Partial Dispersion Ratio Pg,F

Calculated from refractive indices ng, nF and nc.

(5) Deviation ΔPg,F of Partial Dispersion Ratio from Normal Line

Calculated from a partial dispersion ratio Pg,F⁽⁰⁾ on a normal line,which ratio was calculated from a partial dispersion ratio Pg,F and anAbbe's number νd.

TABLE 1 Glass No. 1 2 3 4 5 6 7 8 9 10 11 SiO2 mass % 24.63 24.92 25.3625.82 23.64 22.84 23.43 23.88 23.19 22.95 21.87 mol % 39.73 40.00 40.2740.55 40.00 40.00 39.74 40.00 40.28 40.00 37.00 Nb2O5 mass % 37.58 38.0038.70 39.39 37.85 36.58 37.52 38.25 37.14 36.77 37.63 mol % 13.70 13.7913.89 13.98 14.47 14.48 14.38 14.47 14.58 14.47 14.38 TiO2 mass % 9.039.14 9.30 9.47 9.75 9.42 10.74 10.95 8.50 9.47 10.77 mol % 10.96 11.0311.11 11.19 12.41 12.41 13.70 13.79 11.11 12.41 13.70 Li2O mass % 4.224.27 4.35 4.43 2.03 0.39 1.81 2.25 1.99 1.77 2.22 mol % 13.70 13.7913.89 13.99 6.90 1.38 6.16 7.59 6.94 6.21 7.53 Na2O mass % 3.50 3.994.51 5.05 4.62 4.47 4.58 4.67 4.54 4.49 4.60 mol % 5.48 6.21 6.94 7.697.59 7.59 7.53 7.59 7.64 7.59 7.53 K2O mass % 2.66 2.69 2.74 2.79 7.0311.73 6.97 6.45 6.89 6.82 5.72 mol % 2.74 2.76 2.78 2.80 7.59 13.10 7.536.90 7.64 7.59 6.16 B2O3 mass % 0.98 1.00 1.01 1.03 0.94 0.91 0.94 0.950.93 0.92 0.94 mol % 1.37 1.38 1.39 1.40 1.38 1.38 1.37 1.38 1.39 1.381.37 ZrO2 mass % 5.22 5.29 5.38 5.48 5.01 4.84 4.97 3.38 4.92 4.87 4.98mol % 4.11 4.14 4.17 4.20 4.14 4.14 4.11 2.76 4.17 4.14 4.11 WO3 mass %1.64 3.32 3.37 3.44 6.29 6.08 6.23 6.35 6.17 6.11 6.25 mol % 0.68 1.381.39 1.40 2.76 2.76 2.74 2.76 2.78 2.76 2.74 MgO mass % 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 CaO mass % 0.79 0.80 0.82 0.83 0.76 0.730.75 0.77 0.75 0.74 0.76 mol % 1.37 1.38 1.39 1.40 1.38 1.38 1.37 1.381.39 1.38 1.37 SrO mass % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00BaO mass % 9.75 6.58 4.46 2.27 2.08 2.01 2.06 2.10 2.04 2.02 2.07 mol %6.16 4.14 2.78 1.40 1.38 1.38 1.37 1.38 1.39 1.38 1.37 ZnO mass % 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.19 mol % 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 2.74 La2O3 mass % 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 Gd2O3 mass % 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 Y2O3 mass % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ta2O5mass % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.94 3.07 0.00 mol % 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.69 0.69 0.00 Al2O3 mass % 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 Total mass % 100.00 100.00 100.00100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 mol % 100.00100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00R2O mass % 10.38 10.95 11.60 12.27 13.68 16.59 13.36 13.37 13.42 13.0812.54 mol % 21.92 22.76 23.61 24.48 22.08 22.07 21.22 22.08 22.22 21.3921.22 RO mass % 10.54 7.38 5.28 3.10 2.84 2.74 2.81 2.87 2.79 2.76 2.83mol % 7.53 5.52 4.17 2.80 2.76 2.76 2.74 2.76 2.78 2.76 2.74 Nb2O5 +mass % 46.61 47.14 48.00 48.86 47.60 46.00 48.26 49.20 45.64 46.24 48.40TiO2 mol % 24.66 24.82 25.00 25.17 26.88 26.89 28.08 28.26 25.69 26.8828.08 Nb2O5/ mass % 4.16 4.16 4.16 4.16 3.88 3.88 3.49 3.49 4.37 3.883.49 TiO2 mol % 1.25 1.25 1.25 1.25 1.17 1.17 1.05 1.05 1.31 1.17 1.05(Note 1) Nb₂O₅ + TiO₂ stands for a total content of Nb₂O₅ and TiO₂.(Note 2) Nb₂O₅/TiO₂ stands for a value obtained by dividing the contentof Nb₂O₅ by the content of TiO₂. (Note 3) R₂O stands for a total contentof Li₂O, Na₂O and K₂O. (Note 4) RO stands for a total content of CaO,SrO and BaO. (Note 5) LT stands for a liquidus temperature. (Note 6) Tgstands for a glass transition temperature. (Note 7) Glasses Nos. 1 to 11were optical glasses according to the first embodiment.

TABLE 2 Glass No. 12 13 14 15 16 17 SiO2 mass % 23.85 21.53 19.73 20.7121.75 19.31 mol % 37.50 32.66 33.55 34.94 36.36 33.53 Nb2O5 mass % 39.1033.70 41.82 42.20 42.52 41.00 mol % 13.90 11.60 16.08 16.09 16.08 16.10TiO2 mass % 9.40 16.70 8.70 8.80 8.91 8.60 mol % 11.10 19.01 11.19 11.1911.18 11.20 Li2O mass % 5.49 5.58 2.66 2.68 2.70 2.60 mol % 17.36 17.019.09 9.09 9.09 9.09 Na2O mass % 1.37 1.39 5.51 5.56 5.61 5.40 mol % 2.082.04 9.09 9.09 9.09 9.09 K2O mass % 2.77 2.81 2.58 2.60 2.62 2.53 mol %2.78 2.72 2.80 2.80 2.80 2.80 B2O3 mass % 2.05 2.08 1.91 1.92 1.94 1.87mol % 2.78 2.72 2.80 2.80 2.80 2.80 ZrO2 mass % 5.44 5.52 5.06 3.40 1.724.96 mol % 4.17 4.08 4.20 2.80 1.40 4.20 WO3 mass % 1.70 1.73 1.59 1.601.61 1.55 mol % 0.69 0.68 0.70 0.70 0.70 0.70 MgO mass % 0.00 0.00 0.000.00 0.00 0.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 CaO mass % 2.06 2.091.92 1.93 1.95 0.75 mol % 3.47 3.40 3.50 3.50 3.50 1.40 SrO mass % 0.000.00 0.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 BaO mass %6.77 6.87 6.29 6.35 6.40 9.25 mol % 4.17 4.08 4.20 4.20 4.20 6.29 ZnOmass % 0.00 0.00 2.23 2.25 2.27 2.18 mol % 0.00 0.00 2.80 2.80 2.80 2.80La2O3 mass % 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.00 0.00 0.00 0.000.00 0.00 Gd2O3 mass % 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.00 0.000.00 0.00 0.00 0.00 Y2O3 mass % 0.00 0.00 0.00 0.00 0.00 0.00 mol % 0.000.00 0.00 0.00 0.00 0.00 Ta2O5 mass % 0.00 0.00 0.00 0.00 0.00 0.00 mol% 0.00 0.00 0.00 0.00 0.00 0.00 Al2O3 mass % 0.00 0.00 0.00 0.00 0.000.00 mol % 0.00 0.00 0.00 0.00 0.00 0.00 Total mass % 100.00 100.00100.00 100.00 100.00 100.00 mol % 100.00 100.00 100.00  100.00 100.00 100.00 R2O mass % 9.63 9.78 10.75 10.84 10.93 10.53 mol % 22.22 21.7720.98 20.98 20.98 20.98 RO mass % 8.83 8.96 8.21 8.28 8.35 10.00 mol %7.64 7.48 7.70 7.70 7.70 7.69 Nb2O5 + TiO2 mass % 48.50 50.40 50.5251.00 51.43 49.60 mol % 25.00 30.61 27.27 27.28 27.26 27.30 Nb2O5 mass %4.16 2.02 4.81 4.80 4.77 4.77 NbO2 mass % 1.25 0.61 1.44 1.44 1.44 1.44(Note 1) Nb2O5 + TiO2 stands for a total content of Nb2O5 and TiO2.(Note 2) Nb2O5/TiO2 stands for a value obtained by dividing the contentof Nb2O5 by the content of TiO2. (Note 3) R2O stands for a total contentof Li2O, Na2O and K2O. (Note 4) RO stands for a total content of CaO,SrO and BaO. (Note 5) LT stands for a liquidus temperature. (Note 6) Tgstands for a glass transition temperature. (Note 7) Glasses Nos. 12 to17 were optical glasses according to the second embodiment.

TABLE 3 Glass Specific No. LT (° C.) Tg (° C.) gravity nd νd nc nF ngPg, F. ΔPg, F. 1 1150 567 3.617 1.85110 25.79 1.84161 1.87461 1.894780.611 0.0093 2 1100 556 3.577 1.85005 25.44 1.84044 1.87386 1.894320.612 0.0097 3 1090 554 3.530 1.84748 25.34 1.83787 1.87132 1.891840.613 0.0108 4 1090 550 3.484 1.84475 25.18 1.83512 1.86867 1.889180.611 0.0084 5 1130 561 3.519 1.84054 24.46 1.83070 1.86507 1.886250.616 0.0120 6 1130 577 3.462 1.81321 25.10 1.80389 1.83629 1.856220.615 0.0120 7 1130 562 3.541 1.84591 24.05 1.83585 1.87102 1.892670.616 0.0106 8 1130 554 3.513 1.84600 24.06 1.83588 1.87104 1.892710.616 0.0113 9 1130 563 3.576 1.83991 24.65 1.83013 1.86420 1.885150.615 0.0110 10 1130 560 3.599 1.84905 24.00 1.83893 1.87430 1.896250.621 0.0155 11 1120 548 3.600 1.86214 23.86 1.85179 1.88792 1.910220.617 0.0119 12 1160 547 3.566 1.86077 25.61 1.85111 1.88472 1.905260.611 0.0089 13 1170 546 3.575 1.89283 23.93 1.88215 1.91946 1.942570.619 0.0142 14 1170 549 3.697 1.88093 24.39 1.87056 1.90668 1.928900.615 0.0108 15 1160 542 3.662 1.87372 24.45 1.86348 1.89921 1.921170.615 0.0103 16 1180 539 3.629 1.86630 24.52 1.85617 1.89150 1.913230.615 0.0109 17 1180 539 3.747 1.88192 24.44 1.87159 1.90768 1.929930.617 0.0122 (Note 1) LT stands for a liquidus temperature. (Note 2) Tgstands for a glass transition temperature. (Note 3) Glasses Nos. 1 to 11were optical glasses according to the first embodiment, and Glasses Nos.12 to 17 were optical glasses according to the second embodiment.

Comparative Example

Glass materials prepared so as to have compositions of Examples 1 to 13in JP2004-161598A were melted according to the method described in thesame document. In Examples 1 and 2, melts devitrified during theirstirring, and in Examples 4 to 13, no glass was formed. In Example 3, aglass was obtained by casting a melt into a mold, while theprecipitation of a crystal was observed inside.

Example 2

Raw materials prepared so as to give the same optical glasses as thoseproduced in Example 1 were melted, refined and homogenized to preparemolten glasses, molten glasses were respectively caused to drop from anozzle made of platinum, and drops were received with preform-shapingmolds and shaped into spherical preforms formed of the above variousglasses while they were caused to float by applying gas pressure.

Further, the above molten glasses were respectively caused to flow outof a pipe made of platinum, lower ends of them were received withpreform-shaping molds, narrow portions were formed in the molten glassflows, and then the preform-shaping molds were rapidly moved verticallydownward to cut the molten glass flows in the narrow portions, andseparated molten glass lumps were received with the preform-shapingmolds and shaped into spherical preforms formed of the above variousglasses while they were caused to float by applying gas pressure.

Example 3

The molten glasses prepared in Example 2 were respectively continuouslycast into molds and molded into glass blocks, and then each of the glassblocks was annealed and cut to give glass pieces. These glass pieceswere ground and polished to give preforms formed of the above variousglasses.

Example 4

The molten glasses prepared in Example 2 were respectively continuouslycast into molds and molded into glass blocks, and then each of the glassblocks was annealed and cut to give glass pieces. These glass pieceswere barrel-polished to give press-molding glass gobs formed of theabove various glasses.

Example 5

A carbon-containing film was coated on the surface of each of thepreforms prepared in Examples 2 and 3, and each preform was respectivelyintroduced into a press mold having molding surfaces provided with acarbon-containing release film and including upper and lower moldmembers and a sleeve member. The mold and each preform were heated in anitrogen atmosphere to soften the preforms, and the preforms wereprecision press-molded to produce various lenses formed of the abovevarious glasses, such as aspherical convex meniscus lenses, asphericalconcave meniscus lenses, aspherical biconvex lenses and asphericalbiconcave lenses.

Example 6

A powder release agent prepared from boron nitride was uniformly appliedto the surface of each of the glass gobs prepared in Example 4, and theglass gobs were softened under heat in the atmosphere and press-moldedwith a press mold to produce blanks for various lenses such as sphericalconvex meniscus lenses, spherical concave meniscus lenses, sphericalbiconvex lenses and spherical biconcave lenses. Lens blanks formed ofthe above various glasses were produced in the above manner.

Example 7

Each of the molten glasses prepared in Example 2 was caused to flow out,each molten glass flow was cut with shears to separate molten glasslumps, and each lump was press-molded with a press mold, to producevarious lenses formed of various glasses, such as spherical convexmeniscus lenses, spherical concave meniscus lenses, spherical biconvexlenses and spherical biconcave lenses. Lens banks formed of the abovevarious glasses were produced in the above manner.

Example 8

The lens blanks prepared in Examples 6 and 7 were annealed to remove astrain and adjust their refractive indices to desired values, and thenthey were ground and polished to produce various lenses formed of theabove various glasses, such as spherical convex meniscus lenses,spherical concave meniscus lenses, spherical biconvex lenses andspherical biconcave lenses. Lenses formed of the above various lenseswere produced in the above manner.

Example 9

The molten glasses prepared in Example 2 were caused to flow out andcast into molds to produce glass blocks, these blocks were cut, and theresultant cut pieces were ground and polished to produce variousspherical lenses and prisms.

INDUSTRIAL UTILITY

The optical glass of this invention has a high dispersion property andis suitable for correcting aberration of high order, and it can besuitably used for producing press-molding glass materials such asprecision press-molding preforms, etc., optical element blanks andoptical elements.

1. An optical glass comprising, by mass %, 12 to 40% of SiO₂, 15% ormore but less than 42% of Nb₂O₅, 2% or more but less than 18% of TiO₂,(provided that Nb₂O₅/TiO₂ is over 0.6), 0.1 to 20% of Li₂O, 0.1 to 15%of Na₂O, and 0.1 to 25% of K₂O, the optical glass having an Abbe'snumber νd of 20 to 30, a ΔPg,F of 0.016 or less and a liquidustemperature of 1,200° C. or lower.
 2. An optical glass as recited inclaim 1, which contains, as optional components, 0 to 10% of B₂O₃, 0 to20% of ZrO₂, 0 to 22% of WO₃, 0 to 17% of CaO, 0 to 13% of SrO, 0 to 20%of BaO, (provided that the total content of CaO, SrO and BaO is 0 to25%), 0 to 13% of ZnO, 0 to 3% of La₂O₃, 0 to 3% of Gd₂O₃, 0 to 3% ofY₂O₃, 0 to 3% of Yb₂O₃, 0 to 10% of Ta₂O₅, 0 to 3% of GeO₂, 0 to 10% ofBi₂O₃, and 0 to 10% of Al₂O₃, the total content of Nb₂O₅ and TiO₂ being35 to 65%, the total content of Li₂O, Na₂O and K₂O being 1 to 30%, theoptical glass having a refractive index nd of 1.82 or more but less than1.87.
 3. The optical glass as recited in claim 1, which contains, asoptional components, 0 to 10% of B₂O₃, 0 to 20% of ZrO₂, 0 to 20% ofWO₃, 0 to 13% of CaO, 0 to 13% of SrO, 0 to 20% of BaO, (provided thatthe total content of CaO, SrO and BaO is 0 to 25%), 0 to 13% of ZnO, 0to 3% of La₂O₃, 0 to 3% of Gd₂O₃, 0 to 3% of Y₂O₃, 0 to 3% of Yb₂O₃, 0to 10% of Ta₂O₅, 0 to 3% of GeO₂, 0 to 10% of Bi₂O₃, and 0 to 10% ofAl₂O₃, the total content of Nb₂O₅ and TiO₂ being 30 to 60%, the totalcontent of K₂O being 0.1 to 15%, the total content of Li₂O, Na₂O and K₂Obeing 1 to 25%, the optical glass having a refractive index nd of 1.87to 1.90.
 4. The optical glass as recited in claim 1, which containsSb₂O₃ in an amount of 0 to 2% and SnO₂ in an amount of 0 to 2%, theseamounts being based on a glass composition excluding Sb₂O₃ and SnO₂. 5.The optical glass as recited in claim 1, which has a partial dispersionratio, Pg,F, of 0.580 to 0.620.
 6. The optical glass as recited in claim1, which has a glass transition temperature of less than 600° C.
 7. Apress-molding glass material formed of the optical glass recited inclaim
 1. 8. An optical element formed of the optical glass recited inclaim
 1. 9. A process for producing an optical element blank, whichcomprises softening the glass material recited in claim 7 under heat andpress-molding the glass material.
 10. A process for producing an opticalelement blank, which comprises supplying a molten glass to a press moldand press-molding the molten glass, wherein glass raw materials preparedso as to obtain the optical glass recited in claim 1 are melted underheat and the resultant molten glass is press-molded.
 11. A process forproducing an optical element, which comprises grinding and polishing theoptical element blank produced by the process recited in claim
 9. 12. Aprocess for producing an optical element, which comprises heating thepress-molding glass material recited in claim 7 and precisionpress-molding it with a press mold.
 13. The process for producing anoptical element as recited in claim 12, which comprises introducing theglass material into the press mold and heating said mold and the glassmaterial together.
 14. A process for producing an optical element asrecited in claim 12, which comprises heating the glass material andintroducing the glass material to a pre-heated press mold to carry outthe precision press-molding.